This invention relates to an X-ray exposure apparatus used in the manufacture of various devices, namely semiconductor chips such as IC and LSI chips, display devices such as liquid crystal panels, detector elements such as magnetic heads and image sensing devices such as CCDs, to a reticle such as a mask and to a device manufacturing method using the reticle.
The recent increase in density and operating speed of semiconductor integrated circuits has been accompanied by a decrease in pattern line width of integrated circuits. Semiconductor manufacturing methods also demand much higher performance. For this reason, steppers utilizing shorter and shorter exposure wavelengths, e.g., extreme ultraviolet rays such as KrF lasers (having a wavelength of 248 nm), ArF lasers (having a wavelength of 193 nm) and F2 lasers (having a wavelength of 157 nm) and X-rays (0.2xcx9c1.5 nm), have been developed for exposure apparatuses used in the formation of a resist pattern in the lithography part of the semiconductor manufacturing process.
With exposure using X-rays, an X-ray mask serving as a reticle on which a desired pattern has been formed is brought into close proximity with a wafer serving as a resist-coated substrate, and the X-ray mask is irradiated from above with X-rays to transfer the mask pattern to the wafer.
A method of exposure using a synchrotron light emission has been proposed for the purpose of obtaining high-intensity X-rays in such an X-ray exposure technique, and it has been shown that a pattern can be transferred with a wavelength of less than 100 nm. The synchrotron radiation light source, however, requires elaborate facilities. Though the source is effective in the production of semiconductor devices, it is not suitable for small devices used in prototypes, for example. Accordingly, there has also been proposed an exposure apparatus that employs an X-ray source which is small enough to be usable in prototypes and which generates X-rays of high intensity. One example is referred to as a xe2x80x9claser plasma ray sourcexe2x80x9d, as illustrated in the specification of U.S. Pat. No. 4,896,341. This apparatus irradiates a target with laser light from a laser to generate a plasma, and uses X-rays that are produced from the plasma. Another example generates a pinch plasma by electrical discharge in a gas, and produces X-rays from this plasma, as described in the Journal of Vacuum Science Technology 19(4), November/December 1981, p. 1190.
Regardless of which light source is used, the resolution of the transferred pattern declines because diffraction is utilized in X-ray exposure. The wavelength of X-rays is short and does not cause a decline in resolution. However, it has been found that the decline in resolution becomes a problem as the pattern to be transferred becomes extremely fine.
For example, X-ray intensity distribution on the surface of a wafer is as indicated by the solid line in FIG. 5. The curve is obtained as the result of calculation by Fresnel integration, in which the thickness of the absorbing body was 0.25 xcexcm, the spacing between the X-ray mask and wafer was 10 xcexcm and the mask was irradiated with perfectly collimated X-rays. The mask had a line-and-space pattern of transparent area 90 nm/absorbing area 90 nm. A peak in X-ray intensity appears below the transparent area and at other locations as well. When this pattern is transferred to a negative resist and then developed, the resist at locations where the X-ray intensity is greater than a fixed value remains after development and is resolved as a pattern. The fixed value is considered to be a slice level and is decided by the type of resist, development time, type of developing solution and temperature. In the case of a chemical amplification resist, the fixed value is decided also by the PEB (Post-Exposure Bake) conditions, namely temperature and time.
For example, FIG. 5 illustrates the result obtained by normalizing the X-ray intensity distribution on the wafer surface by the X-ray intensity below a sufficiently large transparent area. It is believed that if the slice level is 1.0, the resist between X1 and X2 will remain after development. Though the width of the resist pattern is, accurately speaking, different from the size L12 (=X2xe2x88x92X1) of the optical image, it will be understood that they approximately coincide, with the value being 66 nm.
Next, the size of the transparent area is gradually changed, the X-ray intensity distribution is calculated and the width of the resist pattern is found from the size of the optical image. This is indicated by the solid line in FIG. 7. Here the width of the transparent area is plotted along the horizontal axis and the size of the resist pattern along the vertical axis. The slice level is changed to 0.8, 0.6 and 0.4, as indicated by the dotted line, broken line and dot-and-dash line, respectively.
However, it will be understood from FIG. 7 that there is a region of transparent areas in which the width of the resist pattern does not necessarily increase but decreases instead and a region in which there is no change in the width of the resist pattern as the mask pattern size, i.e., the size of the transparent area, increases. This indicates that performing exposure using a mask consisting of a mixture of patterns having a plurality of sizes in these regions is difficult. The reason for this is as follows: In this region of mask pattern sizes, X-rays that have passed through the transparent area collect in the diffraction peak and act in a direction that raises X-ray intensity and not in a direction that broadens the width of the X-ray intensity pattern. For example, FIG. 6 illustrates an X-ray intensity distribution in which the size of the transparent area is 220 nm. However, when this is compared with an X-ray distribution (FIG. 5) in which the size of the transparent area is 90 nm, it is seen that the peak intensity is 1.5 times higher in FIG. 6. Since the peak intensity increases more than the ratio of the sizes of transparent areas, the width of the peak decreases rather than increases regardless of the fact that size of the transparent area increases.
If the X-ray dose for which the diffraction peak intensity has risen can be converted by some method to a direction that enlarges the width of the diffraction peak, i.e., if the X-ray intensity distribution can be defocused by a suitable amount, then it should be possible to enlarge the width of the resist pattern along with the size of the mast pattern.
As a means for achieving this, it is considered to change the shape of the mask absorbing area or the X-ray spectrum to thereby change the X-ray intensity distribution on the surface of the wafer. However, such a method is undesirable in that it not only complicates the apparatus but also may make it impossible to obtain the desired pattern.
Another method is to utilize some type of defocusing. Utilizing defocusing means performing a physical operation that corresponds to calculation for convoluting the X-ray intensity distribution on the wafer surface, and obtaining an exposure intensity distribution that differs from the X-ray intensity distribution on the wafer surface in a case where defocusing is not utilized.
In the case of a chemical amplification resist, acid produced by exposure diffuses into the resist to thereby raise sensitivity. Defocusing, therefore, is inherent. It is possible to control this defocusing by changing the conditions of PEB. If the PEB conditions are changed, however, the sensitivity of the resist also changes at the same time and it becomes necessary to determine the proper conditions. Changing the PEB conditions, therefore, is undesirable. Further, in an X-ray exposure apparatus, X-rays that impinge upon the resist produce secondary electrons. Since the secondary electrons diffuse, defocusing is inherent. Since defocusing produced by secondary electrons is decided by the spectrum of the X-rays that impinge upon the resist, it is necessary to change the spectrum for purposes of control. This is difficult.
Accordingly, an object of the present invention is to make it easy to control the defocusing of X-rays on a resist, which is an important factor in the manufacturing process, so as to make it possible to form a more appropriate resist pattern.
Another object of the present invention is to provide an X-ray exposure apparatus that improves process tolerance in terms of exposing finer patterns.
According to the present invention, the foregoing objects are attained by providing an X-ray exposure apparatus comprising: a plasma X-ray source for generating X-rays; a collimator for reducing a global divergence angle of X-rays that diverge from the X-ray source, and irradiating a reticle with the X-rays; and a moving mechanism for moving the collimator.
Preferably, in order to attain the foregoing objects, the local divergence angle of X-rays that impinge upon each point of a mask is changed in order to perform control for optimizing defocusing. Changing the local divergence angle of X-rays that impinge upon each point of a mask is achieved by providing an X-ray exposure apparatus having a plasma X-ray source for generating X-rays by plasma generation, and a collimator for converging X-rays that diverge from the X-ray source and reducing a global divergence angle to irradiate a reticle with the X-rays, wherein a pattern on the reticle is transferred to a substrate that has been coated with a photosensitive agent, the apparatus including a mechanism for changing the position of the collimator.
Further, the foregoing objects are attained by providing an X-ray exposure apparatus having a mechanism for changing the position of a collimator along the axial direction of the collimator or in a direction at right angles thereto.
Further, the foregoing objects are attained by providing an X-ray exposure apparatus having a mechanism for changing the position of a collimator in a direction at right angles to the axis of the collimator during exposure.
Further, the foregoing objects are attained by providing an X-ray exposure apparatus having two or more collimators and a function for exchanging these two or more collimators.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.